Gamma ray dose (rate) meters and dose equivalent (rate) meters are widely used in military, national defense and civil fields, are extremely important tools for guaranteeing the security of nuclear facilities, gamma ray devices, relative workers and the public. To ensure the accuracy and reliability of performance and measured values thereof, they should be verified or calibrated periodically according to the Metrology Law of the People's Republic of China and correlative regulations.
Gamma dose (rate) meters should be verified and calibrated on gamma air kerma secondary standard devices containing secondary standard reference radiation constituted by isotope radiation sources according to the requirements of the national standard GB/T 12162.1-2000 “X and gamma reference radiation for calibrating dose meters and dose rate meters and for determining their response as a function of photon energy—Part 1:—Radiation characteristics and production methods”, GB/T12162.2-2004 “X and gamma reference radiation for calibrating dose meters and dose rate meters and for determining their response as a function of photon energy, Part 2: Dosimetry for radiation protection over the energy ranges 8 keV to 1.3 MeV and 4 MeV to 9 MeV”, and JJG393-2003 “Verification Regulation of X and Gamma Radiation Dose Equivalent (Rate) Meters and Monitors Used in Radiation Protection”. In the process of the verification and calibration work, a secondary standard reference radiation should be verified by using an air kerma measurement standard instrument to obtain the air kerma conventional true value at the point of test of the secondary standard reference radiation; then the reference point disposed on the probe of the dosimeter being verified is accurately positioned in the secondary standard reference radiation as required. And measurement is performed to obtain the calibration factor
      K    =                            K          .                          air          ,          c                                                  M            c                    .                ′              ,wherein, {dot over (K)}air,c is the gamma air kerma (rate) measured or calculated by the standard instrument at the experiment point of the secondary standard reference radiation, i.e., the conventional true value of gamma air kerma (rate) at the experiment point, and
      M    .    cis the indicate value of the dosimeter being verified.
When a gamma air kerma secondary standard device is built, the dimension of reference radiation influencing the dose value, the scattering rays from the shielding wall and the ground, the radiation area of the ray beams, and the non-uniformity of the irradiation area should be designed scientifically, and be tested and verified through detailed experiments, so as to check whether the standard reference radiation meets the requirements. In accordance with relevant standards, the dimension of the standard reference radiation meeting above requirements shall not be smaller than 4 m×4 m×3 m, and the dose rate of gamma rays of the isotope radiation source shall cover the range from μGy/h to mGy/h. Such standard reference radiation cannot be removed no matter in volume or in weight including a shielding building or the like, which leads that all gamma ray dosimeters must be delivered to metrology technology institutions possessing standard reference radiation at fixed sites for verification or calibration. Dosimeters, for the purposes of radiation security monitoring on nuclear power plant reactors and relevant nuclear facilities, are impossible or difficult to be dismounted, and cannot be periodically verified or calibrated by scientific methods and technologies and proper devices yet. Thus it brings hidden danger for radiation security.
One way to realize on-site and in-situ verification or calibration for gamma ray dosimeters is to reduce the spatial volume and the weight of an at least 4 m×4 m×3 m standard reference radiation, which prescribed by the standards, till dismount facilitated. However, reducing the spatial volume of the reference radiation inevitably leads to increase of scattering components in the radiation. Thus the dose contribution rate of the scattered rays in the minitype reference radiation exceeds 5%, which does not comply with the requirements of existing standards, influences response of the dosimeter and results a calibration error.